It's hard not to list classics like Entombed or Asphyx for example. But the obvious will always stand out. Autopsy "Mental Funeral" is another one but sometimes forgotten by the masses. I will just list some that don't make too many lists:

Aeternus "Dark Sorcery" EP. The 2 LPs that followed are also great but this EP is creepy. Music that didn't follow any trends and the most original Norway has ever produced.

Therion "Of Darkness..." Because the band took a totally different path I think this LP has been forgotten.

Disembowelment "Transcendence into the Peripheral". Not too many bands are able to incorporate so much into their music and make it work. One listen to the song "The Tree Of Life" will make most people press the repeat button while most songs that reach 10 minute mark become boring and repetitive. Atmosphere, brutality and controlled chaos all blended together to perfection.

Amorphis "The Karelian Isthmus". Because of the Swedish explosion of Death Metal I think this album was overlooked by many people. It has traits of the Swedish sound but it's also one of those albums that can't really be compared to anything else that was released at that time.